Inherited life span variations in strains of the true slime mold Didymium iridis range from 60 to 180 days. The completion of 3rd. generation backcrosses and an extensive F2 series of crosses should provide enough data to finish the analysis of an apparently complex polygenic system overlaid with several dominant-recessive genes which may be semi-lethals. Cytological investigations will also be made on the effects of polyploidy, both natural age related and constructed strains, on nuclear function and division cycle times. An attempt will then be made to correlate these results with the onset of plasmodial senescence.